1. Field of the Invention
The present invention relates to a gas sensor and particularly, to a gas detector that measures the concentration of a gas using a characteristic infrared absorption band of the gas.
2. Description of the Prior Art
The gas analyzer manufacturing industry has employed a number of gas-detecting techniques in their devices for detecting specific gases. The techniques can be categorized into non-interactive gas analysis and interactive gas analysis. The non-interactive gas analysis techniques include non-dispersive infrared (NDIR) and dispersive infrared (DIR) techniques. Both NDIR and DIR techniques utilize the principle that various gases exhibit substantial absorption at characteristic wavelengths in the infrared radiation spectrum. Thus, a gas analyzer using the NDIR technique often uses a narrow-band transmission filter to isolate a specific wavelength band of infrared light that corresponds to the absorption spectrum of a target gas. In contrast, a gas analyzer using the DIR technique typically includes a prism or diffraction grating to isolate a specific wavelength band.
The non-interactive gas analysis techniques, especially the NDIR technique, offer a number of advantages over interactive gas analysis techniques which often include electrochemical fuel cells, sintered semiconductor (tin oxide), or catalysts (platinum bead) that chemically interact with a target gas. The advantages of non-interactive analysis include fast detection response, gas detection specificity, long term measurement stability, reduced maintenance cost, and good sensitivity. Interactive gas sensors have several drawbacks. The interactive gas sensors can be poisoned or contaminated potentially causing malfunctions that can place human life at risk. Additionally, interactive gas sensors are not good at detecting a target gas because the reagent used to determine the concentration of the target gas may react with other gases that are present in a sample, resulting in a false concentration reading for the target gas.
Despite their functional superiority, the NDIR gas sensors were not initially popular due to their structural complexity and high manufacturing cost. However, over the past several decades, a large number of measurement techniques based upon the NDIR principle have been proposed and successfully demonstrated. An early NDIR gas analyzer included an infrared source, a motor-driven mechanical chopper to modulate the source, a pump to push or pull gas through a sample chamber, a narrow bandpass interference filter, a sensitive infrared detector, and an optical system that focuses the infrared energy from the source onto the detector. U.S. Pat. Nos. 3,793,525, 3,811,776, and 4,578,762, which are herein incorporated by references in their entireties, describe early NDIR gas analyzers. Although these NDIR gas analyzers performed well, their large size, structural complexity, and high manufacturing cost precluded their use in a number of applications.
U.S. Pat. Nos. 4,694,173 and 5,026,992, which are herein incorporated by references in their entireties, describe NDIR gas detection techniques that do not use any moving parts such as mechanical choppers. These NDIR gas sensors that are more rugged, compact, and cost-effective than earlier ones. An attempt to further reduce manufacturing cost and structural complexity produced a low-cost NDIR gas sensor that employs a diffusion-type gas sample chamber. This sensor is disclosed in U.S. Pat. No. 5,136,332, which is herein incorporated by reference in its entirety. The diffusion-type gas sample chamber eliminates expensive optics, mechanical choppers, and a pump for pushing or pulling the gas into the sample chamber.
U.S. Pat. No. 5,136,332, which is herein incorporated by reference in its entirety, advanced the idea of using waveguides or tubular sample chambers in NDIR gas sensors. A waveguide sample chamber has highly reflective inner walls that allow probing radiation emanating from an infrared source at one end of the sample chamber to undergo multiple reflections before reaching an infrared detector at the opposite end of the sample chamber. This NDIR technique does not require the use of any optical components other than a pair of infrared transmitting windows at the ends of the sample chamber.
This design works well for low-cost, rugged and relatively high performance NDIR gas sensors, but has several drawbacks. The simple optical transport mechanism that relies only on multiple reflections decreases the ability to focus radiation sharply on the detectors, resulting in poor signal-to-noise ratio and reducing gas detection sensitivity. Furthermore, this optical system design increases the size of the gas sensor because of the lack of optical focusing components that might shorten the sample chamber path length.
The NDIR gas sensors using waveguide sample chambers expanded the scope of applications and created new potential applications of NDIR gas sensors. Thus, the improvement of NDIR gas sensors continued. For example, U.S. Pat. No. 5,464,983, which is herein incorporated by reference in its entirety, discloses sensor temperature stability improvements. U.S. Pat. Nos. 5,650,624 and 5,721,430, which are herein incorporated by references in their entireties, disclose low-power passive NDIR gas sensors. U.S. Pat. Nos. 5,444,249 and 5,834,777, which are herein incorporated by references in their entireties, disclose NDIR gas sensors fabricated on a monolithic silicon chip.
One important feature for NDIR sensors that has long been overlooked is the intrinsic safety of operating NDIR gas sensors in an explosive environment. The infrared light source in an NDIR gas sensor could ignite a flammable gas inside the sensor. If the ignition escapes from the NDIR sensor, a wider explosion could occur. An intrinsically safe, portable NDIR sensor may open new application areas, such as underground tunnels and sewers, chemical plants, and oil refineries.
An aspect of the present invention provides an NDIR sensor with an efficient configuration. The sensor includes a metallic tube, a platform that fits into the bottom end of the tube, a diffusion filter that fits into a top end of the tube, and an optical system on the platform. The diffusion filter allows a gas to diffuse into and out of a chamber formed in the tube between the platform and the diffusion filter. The platform is typically a printed circuit board, on which optical and electrical systems are mounted, and the diffusion filter is typically a sintered metal and/or plastic fiber filter. The diffusion filter and the platform can be attached to the tube to create an explosion-proof chamber capable of containing an explosion within the chamber.
The optical system typically includes an infrared source, a curved mirror on the inner wall of the tube, and a detector assembly. The curved mirror directs and focuses light from the infrared source onto the detector assembly. The detector assembly receives the infrared light reflected by the mirror and determines the amount of light absorbed by the gas in the tube. Coating a reflecting material on or polishing a portion of the inner wall of the tube can form the mirror in the sensor. The infrared source is typically a miniature light bulb.
The gas sensor may further include a partition between the infrared source and the detector assembly, a removable filter, connecting pins attached to the platform, and a sealing layer formed under the platform. The partition reduces cross-talk or direct transmission between the infrared source and the detector assembly and thereby increases the optical path length through the gas sample to the detector assembly. The connecting pins provide electrical communications between the gas sensor and an external system.
In one embodiment, the detector assembly includes a signal detector and a reference detector. The signal detector uses a first bandpass interference filter that passes a specific wavelength range of the infrared light into the signal detector. The wavelength range of the first bandpass interference filter corresponds to a characteristic absorption wavelength of a target gas for the sensor. The reference detector uses a second bandpass interference filter that passes a wavelength range that the target gas does not absorb. The detector assembly may include multiple signal detectors and a single reference detector or multiple signal and reference detectors to detect multiple gases simultaneously.